Could you provide more details? like maybe links to those websites? Does it always say "Site not found" on the same site or does it sometimes load it and at random times say that? if it says that on random times (one time it loads, another time it tells you server not found) and if the ads you see do not belong to the website you are entering it might be that you have a virus, an Adware... You can make sure by testing out those websites on another computer that you know is clean or try them with different browsers, if the problem is the same on the other computers and browsers then it's not Firefox's bug...